It is deemed beneficial to maximize the synthesis of microbial proteins in the rumen because the amino acid profile of the bacterial proteins is very similar to the amino acid profile required by the host animal, in comparison with the majority of the proteins used as nutrition (NRC, 2001). The rumen microorganisms attain their nitrogen requirements for the synthesis of proteins from a mixture of ammonia, free amino acids and peptides. A large proportion of the digestible diet of a ruminant is converted to the end products by way of microbial fermentation, basically volatile fatty acids (VFA), and the majority of the protein is converted to microbial protein.
Since the bacterial amino acid profile is usually of a better quality than the proteins used as nutrition, the objective in the nutrition of ruminants is to maximize the bacterial growth to promote the synthesis of microbial proteins. The bacterial growth has to be sustained with carbohydrates and nitrogen sources. It has been shown that the bacterial growth is increased with the addition of amino acids and/or peptides, both in the case of cellulolytic bacteria and amylolytic bacteria (Maeng and Baldwin, 1976; Argyle and Baldwin, 1989; Kernick, 1991). It has also been described that fiber digestion is increased with the addition of amino acids (Griswold et al., 1994; Carro and Miller, 1999) and proteins (Cruz Soto et al., 1994) to purely cellulolytic bacteria. In turn, Atasoglu et al. (2001) showed, with pure cellulolytic bacteria cultures, that the incorporation of nitrogen in the form of ammonia in microbial cell nitrogen decreases when the proportion of amino acids is increased in the medium, suggesting that the cellulolytic bacteria also use the amino acids, if available. Similar results have been described when the peptide concentrations are increased; although Atasoglu et al. (2001) state that the cellulolytic bacteria prefer to incorporate the nitrogen from the amino acids against the nitrogen from peptides in their cell nitrogen. Nevertheless, in typical peptide and amino acid concentrations in the rumen, approximately 80% of the cell nitrogen is derived from ammoniacal nitrogen. The addition of branch chain amino acids which will ferment as branch chain volatile fatty acids, and the addition of peptides in the rumen fluid increase the fiber digestion, the production of microbial protein and the growth rates (Russell and Sniffen, 1984; Thomsen, 1985).
The increase of the rumen microbial mass improves the milk production yield (NRC, 2001). As already stated above, it is a common practice in the dairy industry to provide amino acid, peptide or yeast supplements to increase bovine milk production. From among the different peptides used, triptone (which is the peptide obtained from the hydrolysis of casein with the enzyme trypsin) is taken as the reference standard for the addition of nitrogen for bacteria under laboratory conditions.
The majority of animal and vegetable proteins can be hydrolyzed either by enzymatic or acid methods. The enzymatic methods are usually the preferred methods, since no acid residue remains and the amino acids retain their L form.
The use of spray dried hemoglobin (SDH) and dried blood in the dairy industry is well known as a source of proteins which are not hydrolyzed in the rumen and reach the small intestine intact, where they may be digested by the intestinal microflora for the maintenance of the intestinal passage. Nevertheless, they are not used as a protein source to increase the rumen bacterial mass.